This invention relates to an electric rotating machine stator comprising a double casing designed to contain polar grounds fastened to the casing by screws. The invention relates especially to electric rotating machines employed in automobile equipment, particularly electric starter motors. The invention further relates to a method of formation of such double casing.
In starter motors, the polar grounds of the stator, which are used to support induction windings, are generally attached to the inside of the casing by screws. For this purpose, the casing has equidistantly spaced holes around its periphery, and such holes are traversed by the screws that attach the polar grounds. Each hole in the casing has milled in its outer opening a countersink into which fits a countersunk head of the attachment screw.
Boring the holes in the casing and milling chamfered countersinks do not involve particular difficulties when the casing is composed of a single, relatively thick, pipe. It is a different case when, in a manner known in and of itself as described for example, in U.S. Pat. No. 3,200,475, the casing is composed of two thinner pipes positioned and fastened one over the other coaxially. Such arrangement makes the casing easier to handle and produces a considerable savings of materials while maintaining the same magnetic performance. At the positions provided for the screws for connecting the polar grounds, holes are bored in the inner pipe to correspond with other holes bored in the outer pipe. Because of the thinness of the outer pipe of the two-part casing, it is impossible to mill the countersinks for the heads of the polar ground attachment screws in the appropriate manner.